Various high-voltage machines may include a generator for generation of electrical energy, an electric motor, or another piece of electrical equipment having a relatively high rated voltage, more particularly a transformer, a bushing, or a cable. Machines of ever higher power are being developed, for example generators, as advancing technology requires ever higher power density. A high-performance generator, for example a turbo generator, may include a stator with a (stator) lamination stack and a multitude of generator grooves with the generator winding therein.
The main insulation of the generator winding against the lamination stack is under high electrical stress. High voltages arise in operation, and must be dissipated in the insulation volume between the conductor bar under high voltage and the lamination stack at ground potential. This increases the field at the edges of the laminations in the lamination stack and the increase, in turn, causes partial discharges. These partial discharges, when they meet the insulation system, lead to significant local heating. This may gradually break down the organic materials of the insulation system to volatile products of low molecular weight, for example to carbon dioxide.
Many insulation systems include an outer corona shield. In larger generators and electric motors, an outer corona shield is applied directly to the surface of the generator winding insulation. Typically, the outer corona shield comprises carbon black- and graphite-containing tapes or coating materials. For system-related reasons, in such an insulation system, the interface between the outer corona shield and main insulation cannot be produced in completely pore-free form. Therefore, with sufficiently high electrical field strengths in the insulation system, there is correspondingly high electrical partial discharge activity, which completely burns out the outer corona shield over time. This leads to premature aging of the insulation and in the worst case to a ground fault of the electrical machine. Such a fault regularly means irreparable complete failure of the machine.
The outer corona shield must have a certain square resistance within a particular range. If it is too low, the lamination stacks can be electrically short-circuited, leading to high induced circulating currents occurring across the ends of the lamination stacks and lead to high-current arcs. In the case of excessive resistance, by contrast, there can be high-voltage spark erosion. Ideally, the resistance in the outer corona shield would be adjustable, such that it would be possible to establish anisotropy, showing elevated conductivity in the radial direction, i.e., from the current conductor toward the lamination stack, and elevated resistance, i.e., low conductivity, in bar direction.